The central visual pathways of amphibians have been chosen as a model system for investigation of the neural substrates of visuomotor behaviors mediated via mesencephalic circuits. In recent years, increased attention has been directed toward the pretectual and accessory optic pathways and their associated correlates of visually guided behaviors in anurans, including optokinetic nystagmus, visual orientation and prey-acquisition behaviors. Our recent studies have utilized neuropharmacological and microiontophoretic analyses as well, demonstrating that neuropharmacologic manipulation of accessory optic and pretectal optic systems can provide a powerful tool with which to elucidate the physiological interactions which occur both within and between these two extratectal optic circuits. The studies proposed in this application are presented within the context of our current knowledge and previous work concerning the accessory opticpretectal circuitry and with regard to relevant histochemical observations: (1) singleunit analysis of the effects of cholinergic drugs in the pretectal nucleus lentiformis mesencephali and the extent to which visually responsive units may be modulated by simultaneous microiontophoresis of cholinergic agonists or antagonists, (2) GABAnergic effects on visual neuronal responses on nLM and the accessory optic nucleus, nBOR, as well as the effects of accessory optic deafferentation on visual responses in the pretectal nLM. These studies will advance our knowledge concerning the functional organization of the accessory optic and pretectal optic nuclei including the neuropharmacologic mechanisms and neuromodulatory processes which ultimately affect visuomotor responses mediated through these extratectal optic systems.